Coating and the printing of labels on small metal cans is a huge volume operation throughout the beverage industries. Under present technology and in a typical installation, the cans are coated or printed with a liquid composition and passed through what is known as a "pin oven" wherein the cans are subjected to air at temperatures, such as 600.degree. F., while being carried through an upward and downward zig-zag path on a chain equipped with horizontally extending can-supporting pins spaced along a chain much as described in U.S. Pat. No. 3,381,391. Typically, the cans are carried by a chain and pin assembly into and through an oven from a printing or coating machine by chain-propelling mechanism which is mechanically connected with the printing machinery to be synchronized therewith to start up, operate and stop together. Under the rapid air movement conditions within the oven, the air is directed primarily against the closed ends of the cans to assure that they remain on the pins. Air cannot to any appreciable extent be directed interiorly of the cans through their open ends. Furthermore, air cannot be directed essentially laterally toward the can to achieve good heat transfer since such movement would tend to flutter, and even dislodge the cans from respective pins. In any event, transfer of heat from the air to the can body is affected in an imperfect manner primarily along a can's outer surface.
An important disadvantage of this prior art system is that any stoppage of the chain-and-pin type of can-transfer system, usually because of a malfunction in the label-printing or coating apparatus, results in overheating of the cans which are stalled in the coat-curing oven. In a typical installation, a single stoppage involves the loss of at least a few dozen cans. Another disadvantage of the so-called "pin oven" is the length of the chain path due to limitations in the air-to-can heat transfer rate that can be achieved. When higher processing speeds are attempted, the length of the oven must be extended.
It has long been recognized that the "pin oven" achieves unsatisfactory use of the heat supplied to the oven and that there is a need for a coating-baking process that achieves higher rates of heat transfer to the workpieces. The pin-and-chain conveyor is obviously disadvantageous for baking inside coatings of cans because the pins must contact an interior surface, and also because heat transfer from exterior air to an interior coating involves the passage of heat through the can wall.
Current advances in the art of coating and baking finishes on cans indicate that it will soon be commercially feasible to coat cans both inside and outside and then pass the cans with the coatings in an initially wet condition through a suitable oven which can simultaneously harden the coatings at approximately equal rates without the coatings being in marring contact with any supporting means. Of immediate need is an oven which can efficiently heat and cure interior coatings without subjecting the outer surfaces of the cans to handling while the cans are in the oven. The term "cans" is used herein for brevity and for all types of containers adaptable to processing by apparatus disclosed herein.
Hence, objects of the invention are: to provide ovens for drying or curing coatings of containers, such as beverage cans, which achieve high heat transfer rates to the work and are thus economical in the consumption of fuel; to provide ovens of which the conveyors may be operated independently of container movements through other can-treating apparatus, such as labeling or coating machinery; to provide ovens which have the capability of efficiently curing interior coatings of cans, the exterior coating of the cans, and when coated both interiorly and exteriorly, curing both coatings simultaneously; provide coating-curing ovens which may be compactly arranged with short work paths while utilizing high temperatures, such as 800.degree. F. or more, within an unusually uniformly heated oven atmosphere; and along with other objects, to provide ovens in which the heated gaseous medium may be directed from all directions at the exterior surfaces of the cans to achieve air impingement against lateral surfaces as well as end surfaces of the cans.